As a matter of fact Daywize build such HR sollution for clients. Look at our website to get some info what HR modules we already build. We must say that we are still thinking about a multi tenant site where multiple customers could be served this way. It can be done but since you are working with sensitive data creating such environment can be a challenge. Because you would never never never come in the situation that somebody had access to the data of another customer of you. That would probably be the end of your business.
But that said, it can be done.
I've used mendix for 4 months now and I think that if you want to create a web based app. for HR it is definitly a good choice. The most difficult part for me with mendix was that you have to think different. but of course I don't have that much experience with mendix as many others but thats my opinion.